Leaf senescence occurs at the end of the plant life cycle, and is controlled by a number of internal factors such as plant hormones and developmental stage, and can be accelerated by environmental stresses (Lim, Kim et al. 2007). Ethylene is a plant hormone with a key role in control of leaf senescence. Application of exogenous ethylene can induce leaf senescence, and suppression of ethylene biosynthesis or response can delay leaf senescence (Lim, Kim et al. 2007). Leaf senescence that has been delayed by reduced ethylene levels or response will eventually be initiated, and it will then proceed normally.
An example of the role of ethylene on crop productivity in field conditions was reported for soybean. Stressful high temperature conditions will reduce yield in soy and other crops. Ethylene production was shown to increase in soybean leaves in response to high temperature, and triggered premature leaf senescence (Djanaguiraman and Prasad 2010). However, prevention of ethylene response by use of the inhibitor 1-methyl cyclopropene (1-MCP) in plants grown at high temperature reduced leaf senescence and allowed increased yield compared to untreated controls also grown at high temperature. 1-MCP is a gas, and not appropriate for application in open fields. Therefore, other methods to delay leaf senescence are needed.
Ethylene also plays a major role in the ripening of fruits and vegetables. Ripening is an important phase of fruit development involving changes in fruit cellular metabolism leading to the development of a soft and edible ripe fruit with desirable quality. However, the softening that accompanies excessive ripening increases postharvest losses and reduces the shelf life of fruits and vegetables during handling, transportation, and storage. Ethylene, as a fruit ripening phytohormone, triggers many events of cell metabolism including initiation of ripening and senescence in fruits and vegetables, particularly in climacteric fruits (reviewed by (Lin, Zhong et al. 2009; Bapat, Trivedi et al. 2010)). Two systems, system 1 and 2 of ethylene production have been described in plants (McMurchie, McGlasson et al. 1972). System 1 operates during normal growth and development and during stress responses while system 2 functions during floral senescence and fruit ripening. System 2 is autocatalytic and it is stimulated by ethylene. Manipulation of ethylene biosynthesis, especially the system 2, in fruits and vegetables can be of strategic importance to enhance the shelf life and to reduce the postharvest losses of these crops.
The ethylene signal transduction pathway is relatively well understood (Lin, Zhong et al. 2009). Ethylene is perceived by a family of two-component histidine kinase-like receptors encoded by the ETHYLENE RESPONSE 1 (ETR1) and related genes. The CONSTITUTIVE TRIPLE RESPONSE 1 (CTR1) gene encodes a Raf-like serine/threonine kinase that has been shown to interact with ethylene receptors. ETHYLENE INSENSITIVE 2 (EIN2) is a positive regulator of ethylene signaling and has been placed downstream of the ethylene-receptor/CTR1 complex. The N-terminus of EIN2 has homology with NRAMP ion transporters, but its exact function in ethylene signaling is not understood. Further downstream of EIN2 are transcription factors, including EIN3-encoded proteins, that regulate genes in response to ethylene.
Mutants of EIN2 have delayed senescence in Arabidopsis, without an effect on flowering time or development (Aeong Oh, Park et al. 1997).
Ethylene insensitivity has been shown to provide improvements in tolerance to certain diseases in certain plants as well as reduced disease tolerance to certain diseases in certain plants (van Loon, L. C., et al. Trends in Plant Sci. 11(4): 184, 2006).